It's almost certainly a watering problem yes, although both over-watering and under-watering will potentially cause the same appearance - the actual problem is root damage in either case. The Acer is dropping it's foliage in order to preserve itself and to prevent dessication. So long as your plant still has viable, undamaged growth buds then it will be capable of recovering once the correct watering regime has been established.

is it in a pot i wonder ?

Sounds like it might well be....

One of our Japanese Acers turned up it toes and died... no apparent reason treated in exactly the same way as all the other Acers in the garden! We removed today and still can't see any reason... I suspect that the tree heard himself say we needed to move - went into shock and died!

Thanks for your helpful replies everyone

lol moon grower one of mine isnt so happy next to 2 that are doing amazingly well.